1. Field of the Invention
The present invention relates to an electric appliance using a luminescent device which is formed by making a semiconductor element (element using a semiconductor thin film) on a substrate using a luminescent element having a film containing an organic compound with which luminescence or phosphorescence is obtained by impressing an electric field, representatively, EL (electroluminescence) display device and its EL display device as a display section.
It should be noted that in the present invention, a luminescent element is referred to an element in which an organic compound layer is provided between a pair of electrodes, a luminescent device is referred to an image display device or a luminescent device using a luminescent element. Moreover, it is defined herein that the category of luminescent devices includes all of a module in which a connector, for example, a Flexible Printed Circuit (FPC), a Tape Automated Bonding (TAB) or a Tape Carrier Package (TCP) is attached to a luminescent element, a module in which a print-wiring board is provided on the tip of the TAB or TCP, or a module in which an IC (integrated circuit) is directly mounted on a luminescent element by a Chip On Glass (COG) method.
2. Related Art
In recent years, the technology for forming a TFT on a substrate has been largely advanced, and the development applied to an active matrix type display device has been proceeded. Particularly, since electric field effect mobility of a TFT using a polysilicon film is higher than that of the conventional TFT using an amorphous silicon film (which is also referred to as “mobility”), an operation at a high speed is capable of being carried out.
Such an active matrix type display device now attracts a great deal of attention since it obtains a variety of advantages such as the reduction of the manufacturing costs, the miniaturization of a display device, the raising of yield, the reduction {increase?} of throughput and the like by utilizing a method in which a variety of circuits and elements are made up on the same substrate. A luminescent element using an organic compound having the characteristics such as being thin type, light weighted, having a high speed responsibility, direct current low voltage drive and the like as an emitter is expected to be applied to the next generation flat panel display. Particularly, a display device in which luminescent elements (which is also referred to as EL element) are arranged in a matrix shape (hereinafter, referred to as “active matrix type EL display device”) is considered to be advantageous when comparing to the conventional liquid crystal display devices from the viewpoint that it has a wide angular field of view and is excellent in visibility.
An active matrix type EL display device provides a switching element consisted of a TFT (hereinafter, referred to simply as switching element) at each pixel, makes a drive element for performing a current control by its TFT for switching (hereinafter, referred to as TFT for current control) operate and makes an EL layer (strictly referring to it, it is an emitting layer) emit. For example, an EL display device described in Japanese Unexamined Patent Publication No. H10-189252 gazette is known.
As for an active matrix type EL display device, two kinds of structures are considered from the viewpoint of emission direction of the light. One is a structure in which the light emitted from the EL element transmits through the opposed substrate and is irradiated into the observer's eyes. In this case, the observer can recognize the image from the opposed substrate side. Another one is a structure in which the light emitted from the EL element transmits through an element substrate and is irradiated into the observer's eyes. In this case, the observer can recognize the image from the element substrate side.
In the case where an active matrix type EL display device was intended to prepare, after a thin film transistor (hereinafter, referred to as TFT) was formed on the insulating surface, an interlayer insulating film is formed on the TFT, an anode of a luminescent element electrically connected to the TFT via the interlayer insulating film is formed, and further on the anode, an organic compound layer is formed, and further, after the organic compound layer was formed, a cathode is formed whereby a luminescent element is formed.
As a material used for a cathode, it is said that it is preferable to use a metal having a small work function (representatively, metal elements belonging to I group or II group of the periodic table) or an alloy containing these. Since the smaller the work function is, the more the luminous efficiency is enhanced, it is preferable that among these, as a material used for a cathode, an alloy containing Li (lithium), which is one of alkaline metals, is used.
However, in the case where a material containing an alkaline metal is used for a cathode, while it can contributes to the enhancement of the luminous efficiency of the luminescent element, there is a fear of the alkaline metal ion used for the cathode being diffused to; be mixed into an active layer of the TFT.
In a TFT, when the voltage is applied to a gate electrode, depending on its polarity, an impurity ion of an alkaline metal or the like is attracted to an active layer side. Then, in the case where these impurity ions cannot be blocked by an insulating film for covering the active layer, these are mixed into the interface between the insulating film and the active layer and into the active layer, causes the increase of interface level and becomes trapping center of a carrier, it is considered to cause the variation of electric characteristics of the TFT and the lowering of the reliability for the TFT.
At present, as an insulating film for covering an active layer of TFT, an inorganic insulating film represented by a silicon oxide film, a silicon nitride film, a silicon oxynitride film and the like and an organic resin film represented by a polyimide film, an acrylic film and the like are used. The experiment for confirming the blocking effects of these insulating films were carried out.
As a result of examining the characteristic variation of the MOS by preparing a MOS on a substrate having an insulating surface and forming an Al—Li alloy via an insulating film (silicon nitride film, silicon oxynitride film) located above the MOS, the characteristic variation was large, and it is considered that the cause of it is mainly attributed to the fact that Li has been mixed into the active layer.
Therefore, it is considered that the variation of TFT characteristics and the lowering of the reliability are also occurred in the case where an EL element having a cathode containing an alkaline metal on TFT was formed.
From the results described above, an insulating film provided between a TFT and an EL element was not sufficient for preventing an impurity ion (representatively, alkaline metal ion) from diffusing from the EL element into the TFT.
Moreover, also in the case where a material containing an alkaline metal in an organic compound layer was used, it is considered that the diffusion of an impurity ion (representatively, alkaline metal ion) from the EL element into the TFT is generated.
Moreover, although there are some cases where what is called a buffer layer is formed between a cathode and an anode, but also in the case where a material containing an alkaline metal in this buffer layer was used, it is considered that the diffusion of the impurity ion (representatively, alkaline metal ion) from the EL element to the TFT is generated.
Moreover, also in the case where a material containing an alkaline-earth metal (which is also referred to as alkaline earth) in a cathode, an anode, a buffer layer, or an organic compound layer was used, similarly, there is a fear of the diffusion of an impurity ion (representatively, alkaline-earth metal ion) from the EL element to the TFT being generated and causing the variation of characteristics of the TFT.